While subscribing to the VStream API you need to specify the shards from which to stream events. While streaming it is possible that the underlying keyspace is resharded. Thus some or all of the shards which were specified may be replaced by new shards after the resharding is completed.
Stream migration logic within VReplication handles this transparently within VTGate. Event sending will be put on hold momentarily during the actual cutover (when writes are switched) and you will start getting the events (and vgtids) for the new set of shards once the cutover is completed.
Here is a sample session using the scripts from the local example.
Run the steps up to and including 205_clean_commerce.sh. Now start a vstream api client in a separate terminal to stream events from the customer table in the customer keyspace, which is currently unsharded.